Orthogonal frequency-division multiplexing (OFDM) is used to perform digital modulation to transmit a data over a transmission channel using a multi-carrier signal. To date, OFDM is used to transmit data in digital communication systems such as asymmetric digital subscriber line (ADSL), wireless local area networks (WLAN), digital video broadcasting—terrestrial (DVB-T), etc. In OFDM data transmission, the available transmission channel is divided into a number of sub-channels, each with its own carrier, or sub-carrier, frequency chosen so that the frequencies are orthogonal to each other. As a result, data streams modulated with these carriers are ideally not subject to inter-channel interference (ICI), also called cross-talk. Modulation of the bit-stream to be transmitted is generally performed using standard modulation schemes such as quadrature amplitude modulation (QAM), phase-shift keying (PSK), etc. The binary bit stream to be transmitted is sub-divided into several bit streams of a certain length. Each of these smaller bit streams, for example groups of 4, 16, or even 1028 bits, is used to modulate a sub-carrier in a predefined manner, using amplitude and/or phase modulation to give a ‘data symbol’ in the resulting multi-carrier signal that is transmitted in its entirety. A complete set of modulated carriers is called a multi-carrier symbol, for example an ‘OFDM symbol’. The number of carriers is predefined, for example 2K OFDM symbol has 2048 sub-carriers and could theoretically therefore transmit that many data symbols. Similarly, 4K OFDM and 8K OFDM could theoretically transmit up to 4096 or 8192 data symbols respectively. Broadcasting standards such as IEEE 802.11a define the rate at which an OFDM symbol is transmitted, i.e. the data rate, for a number of modulation schemes and the corresponding number of bits per symbol. A single OFDM symbol can have a duration of 1120 μs, i.e. the signal is ‘in the air’ for that length of time.
Typically, not all of the carriers in an OFDM symbol are modulated with data, or payload. In order to simplify reception of the signal being transmitted, certain predefined signals are transmitted with each block. For instance, some bandwidth efficiency is sacrificed so that ‘guard intervals’ can be inserted between multi-carrier symbols, while ‘pilot’ signals are periodically inserted at specific carriers for use during the synchronization and equalization phase. By spacing the pilots sufficiently close in the multi-carrier signal, correct channel estimation and interpolation are made possible for the received signal, even in the case of long echoes, for which the density of the fading in the spectrum becomes high.
Transmission channel estimation for a received signal is usually simply referred to as equalization, which is generally quite straightforward as long as the transmitter and receiver are essentially stationary with respect to one another, and channel estimation is generally achieved in a ‘static’ equalizer by performing a time-domain interpolation on certain pilots in the received signal, thus determining the corresponding sub-carriers, and then performing a frequency-domain interpolation to determine the remainder of the sub-carriers. Such a static equalizer requires a sequence of complete multi-carrier symbols to estimate the channel correctly. It is necessary to be able to perform accurate channel estimation on the received signal, i.e. to identify the carriers, in order to be able to determine the data payload, even if the signal arriving at the receiver has been subject to attenuation and multipath (echo). OFDM works optimally in static environment, i.e. when the transmitter and equalizer are essentially stationary with respect to each other. This is generally the case for transmission systems using ADSL, DVB-T, WLAN, etc. Low velocities are tolerable without a noticeable degradation in performance. However, at higher speeds, for example when a user of a hand-held device (such as a mobile phone or pocket personal computer) is receiving a television broadcast while in a car or train, the received signal will be further distorted to a greater or lesser extent owing to the Doppler effect—i.e. the wavelengths of the carriers will be ‘compressed’ or ‘stretched’, depending on the direction and speed of travel of the user with respect to the transmitter. The resulting perceived shift in the carrier frequencies results in a corruption of their orthogonality, which is increased even further by the time-interpolation, resulting in failures in the channel estimation process. As a result, the user's device may fail to regain the originally transmitted data from the received signal.
The use of devices with digital video broadcasting—handheld (DVB-H) capability is becoming more popular, and various attempts are being made to address the problem of Doppler shift in the received OFDM signal. Since time-domain interpolation cannot be used reliably in a fast-changing environment, a frequency-domain analysis of the received signal can be performed to estimate the channel. Frequency-domain interpolation alone, without time-domain interpolation, is better in a mobile environment since no assumptions need be made regarding the temporal stability of the channel. Therefore, some prior art systems use both methods, and determine a point at which a switch-over should be made, as appropriate. However, state of the art solutions that are based on a switchover between time-domain and frequency-domain equalization are not fast enough in reacting to a rapid increase in velocity between the transmitter and receiver, since they require a large number of consecutive symbols in order to arrive at the decision to make the switch-over. Furthermore, these state of the art solutions are generally quite complex at the receiver end.